Chromosomal Problems Affect Nearly All Human Embryos

http://www.sciencedaily.com/releases/2009/07/090701082919.htm

Discovery May Explain Low Fertility Rates In Humans
ScienceDaily (July 1, 2009) — Amsterdam, The Netherlands: For the first time, scientists have shown that chromosomal abnormalities are present in more than 90% of IVF embryos, even those produced by young, fertile couples. Ms Evelyne Vanneste, a PhD student in the Centre for Human Genetics and the University Fertility Center, Leuven University, Belgium, told the 25th annual conference of the European Society of Human Reproduction and Embryology today (Wednesday July 1), that the surprising finding meant that current techniques used in preimplantation genetic screening (PGS), where embryos are screened genetically in order to select the best embryo for transfer, do nothing to improve pregnancy and live birth rates. Indeed, it can lead to potentially viable embryos being discarded, she said. Indeed, it can lead to potentially viable embryos being discarded, she said.

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"Although in vitro culture conditions are known to have a limited influence on the rate of chromosomal imbalances in IVF/ICSI embryos, it is probable that the chromosome instability observed in vitro also occurs in spontaneous pregnancies since, at most, 30% of human conceptions result in a live birth and more than 50% of spontaneous abortions carry chromosomal aberrations. The high rate of chromosomal abnormalities is almost certainly responsible for the low fecundity of humans compared with other mammals," she added.

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"Although PGS is promoted as a way of increasing the chances of a successful pregnancy," said Ms Vanneste, "there has never been any significant evidence that it does, in fact, increase live birth rates after IVF. Our findings have shown that almost every cell of a human embryo carries a different genetic composition; consequently, the one cell that is analysed genetically is not representative of the rest of the embryo. If the tested cell is genetically abnormal, the embryo will not be transferred. But the rest of the embryo might be normal and develop into a healthy person. Therefore, the use of PGS means that potentially viable embryos will be discarded. The prevalent chromosomal instability in all early human IVF embryos explains the failure of PGS to improve the live birth rate per embryo transferred.